Materials having high dielectric constants have been researched for use as dielectric layers for thin film capacitors. As microelectronic circuits have become increasingly integrated, the demand for smaller components has become stronger. The quest for miniaturization is particularly ardent with regard to DRAM cell devices. The migration of integrated circuits to smaller feature sizes is promoting interest in the development of thin film dielectrics having dielectric constants (.epsilon.) greater than those of previously used materials. Typically, films of a-SiO.sub.x have been used as a dielectric material in DRAM capacitors or capacitors of integrated-circuit devices. As the cell size has shrunk, however, designers have resorted to use of extremely thin a-SiO.sub.x films, but these films are problematic as they exhibit a decreased reliability due to finite breakdown fields. Thus, efforts have been directed toward developing new dielectric materials which can be substituted for a-SiO.sub.x, films, thus avoiding the inherent limitations of those films.
Attention has been focused on materials with high values for the dielectric constant (.epsilon.) and figure of merit, the figure of merit (FOM) being the multiple of the dielectric constant (.epsilon.) and breakdown field (E.sub.br) of a material. In other words, the dielectric constant (.epsilon.) times the breakdown field (E.sub.br) [MV/cm] equals its figure of merit (.epsilon.E.sub.br) [.mu.C/cm.sup.2 ]. The figure of merit is a useful unit of measure of the efficacy of a dielectric material because it corresponds to the maximum charge that may be stored on a capacitor and does not depend upon film thickness. In general, one has a wide design flexibility in varying the thickness of the dielectric films used in integrated circuit applications. For example, in high-quality deposited SiO.sub.2 films, the dielectric constant is about 3.7, the breakdown field (E.sub.br) is about 8 MV/cm, translating to a FOM of about 2.6 .mu.C/cm.sup.2.
Titanium-oxide (TiO.sub.2) has a high dielectric constant, making films of TiO.sub.2 potentially useful in various roles in integrated circuits, such as metal oxide semiconductor or memory capacitors, gate oxides, and other circuit elements. See, e.g., Y. H. Lee, K. K. Chan, and M. J. Brady, "Plasma Enhanced Chemical Vapor Deposition of TiO.sub.2 In Microwave-Radio Frequency Hybrid Plasma Reactor," J. VAC. SCI. TECH. A 13 (3) 1995, at p. 596; J. Yan, D. C. Gilmer, S. A. Campbell, W. L. Gladfelter, and P. G. Schmidt, "Structural and Electrical Characterization of TiO.sub.2 Grown From Titanium Tetrakis-Isopropoxide (TTIEP) And TTIP/H.sub.2 O Ambients," J. VAC. SCI. TECH. B 14 (3) 1996, at p. 1706.
In the past, TiO.sub.2 films have been prepared by a variety of processes, including metalorganic, low pressure, and plasma-assisted chemical vapor deposition (e.g., MOCVD, LPCVD, and PCVD), plasma oxidation, and reactive sputtering. Dielectric constants between 25 and 75 generally have been reported for pure x-TiO.sub.2 films. However, x-TiO.sub.2 films have demonstrated high leakage currents (low breakdown fields), which directly and adversely influences the operation of DRAM circuits and impacts on the reliability of the capacitors. Few studies have reported that the leakage current densities are a function of electric fields, but generally it is believed that high leakage currents arise from defects in the films, especially oxygen vacancies. Efforts thus have been directed toward reducing defects (and particularly oxygen vacancies) in the TiO.sub.2 films to thereby decrease the leakage currents. Two approaches have dominated these efforts: (1) optimizing the deposition conditions, and (2) annealing the films. See S. C. Sun and T. F. Chen, "Effects of Electrode Materials and Annealing Ambients on the Electrical Properties of TiO.sub.2 Thin Films by Metalorganic Chemical Vapor Deposition," JPN. J. APPL. PHYS. 36 (1997), part I, No. 3B, at p. 1346. Nonetheless, TiO.sub.2 films having sufficiently low leakage currents for reliable use in DRAM devices presently have not been obtained.
Another approach recently taken to decrease leakage currents or otherwise improve the dielectric properties of x-TiO.sub.2 films has been to incorporate foreign cations into the crystalline films. For example, known composites have included crystalline barium strontium titanate {x-(Ba,Sr)TiO.sub.3 }, described in U.S. Pat. No. 5,552,355, for "Compensation of the Temperature Coefficient of the Dielectric Constant of Barium Strontium Titanate," assigned to the present assignee and incorporated herein. Incorporating barium and strontium-containing dielectric materials into integrated circuit devices has proved problematic, however, as these materials are not sufficiently compatible with the silicon components of the devices. Another known composite comprises perovskite-titanite films such as PbTiO.sub.3, which also may be doped with zirconium (Zr) to yield the ceramic (Pb,Zr)TiO.sub.3, known as PZT. See also E. S. Ramakrishnan, Kenneth D. Cornett, Gary H. Shapiro, and Wei-Yean Howng, "Dielectric Properties of Radio Frequency Magnetron Sputtering Deposited Zirconium Titanate-Based Thin Films," J. ELECTROCHEM SOC., 145 at 358-62 (January 1998) (discussing zirconium-titanate based films). PZT also has been doped with lanthanum in an effort to reduce leakage current, the composite being PZLT. See S. B. Desu, "Minimization of Fatigue In Ferroelectric Films," PHYSICA STATUS SOLIDI A (Oct. 16, 1995), Vol. 151, no. 2, at p. 467-80.
Study of titanium-oxide ceramics for use as dielectric materials as discussed above has focused on the crystalline and polycrystalline phases of the materials, as opposed to amorphous films. In fact, a relative decrease in the dielectric constant for amorphous titanium-oxide films is reported in O. Nakagawara, Y. Toyoda, M. Kobayashi, Y. Yoshino, Y. Katayama, H. Tabata, and T. Kawai, "Electrical Properties of (Zr,Sn)TiO.sub.4 Dielectric Thin Film Prepared by Pulsed Laser Deposition," J. APPL. PHYS. 80, 388 (1996) ("Nakagawara"), which attribute this decrease to the ionic polarizahility of the materials. Nevertheless, the applicant has studied amorphous films of titanium-oxide as dielectrics in the continuing search for new materials and methods for incorporation into a DRAM cell or integrated circuit device. Amorphous titanium-oxide compositions exhibiting usefull properties as dielectrics are described in co-pending U.S. patent application Ser. No. 08/936,132, entitled "Dielectric Materials of Amorphous Compositions and Devices Employing Same," filed Sep. 24, 1997, by Schneemeyer and Van Dover (the inventor herein) ("the '132 application"), assigned to Lucent Technologies, the assignee herein, which is hereby incorporated by reference. The compositions of the '132 application involve amorphous titanium-oxide films doped with both tin (Sn), and either hafnium (Hf) or zirconium (Zr). Therefore, two separate dopants are used. To simplify the process of preparing the dielectric materials, it would be advantageous to discover a composition involving fewer dopants.
Use of the praseodymium (Pr) in titanium oxide films is discussed in V. D. Kushkov, A. M. Zaslavskii, and A. V. Mel'nikov, "Phase Relations and Structural Characteristics of Pr--Ti--O Films," INORGANIC MATERIALS (December 1991), Vol. 27, No. 12, at pp. 2293-5 (translation of Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy (December 1991), Vol. 27, No. 12, at pp. is 2671-2). Kushkov et al. discuss whether these films are crystalline or amorphous as a function of the temperature use to prepare the films, and they do not discuss electrical properties. In M. Gartner, C. Parlog P. Osiceanu, "Spectroellipsometric Characterization Of Lanthanide-Doped Films Obtained Via The Sol-Gel Technique," THIN SOLID FILMS (Oct. 25, 1993), Vol. 234, No. 1-2, at pp. 561-66 titanium-oxide films doped with lanthanum (La), europium (Eu), samarium (Sm) or gadolimiim (Gd) are discussed in terms of altering the structure or optical properties the films, but they do not address whether these dopants may decrease the leakage current or improve the materials' dielectric properties.
As may be appreciated, those concerned with the development of integrated circuit devices continually search for new materials and methods for improving device performance as the circuit size becomes progressively smaller. This search includes the discovery of new dielectric materials compatible for use in DRAM cells or silicon-chip integrated circuit devices having high dielectric constants and large breakdown fields (low leakage currents).